Hybrid vehicles include an engine and an electric machine that operates as a motor/generator with an associated battery to provide an electric vehicle (EV) mode using only electric power to propel the vehicle, or a hybrid electric vehicle (HEV) mode that uses the engine and motor to propel the vehicle. Hybrid vehicles may start the engine using a dedicated starter motor and/or various types and sizes of electric machines that may function as a motor/generator or an integrated starter-generator (ISG). The engine may be started frequently under various operating conditions to meet driver demanded torque or to transition between EV and HEV operating modes such that the selected starting strategy may significantly impact overall vehicle efficiency, performance, and drivability.
The electric machine of a hybrid vehicle may be coupled to the engine to start the engine under various operating conditions. The torque available to start the engine using the disconnect clutch to couple the electric motor is limited based on the size and capabilities of the electric motor. This motor has a limit to the amount of torque it can provide for engine starts, as it also needs to provide propulsive torque. As such, engine starting torque may be reserved by the electric motor to provide sufficient torque capacity during operation to crank the engine and overcome engine friction and inertia, which may vary based on operating conditions such as temperature, and engine stop position, for example, which limits the available motor torque to propel the vehicle. Some engine stopping positions, i.e. positions of pistons with the engine cylinders and associated state of intake/exhaust valves may require engine cranking torque that exceeds the available motor torque, which may result in engine starting issues.